Dielectric optical switch

ABSTRACT

A novel optical filter structure for selectively blocking radiation of predetermined wavelength is described which comprises a plurality of alternate first and second layers deposited on a substantially transparent substrate, the first layers comprising a transparent nontransitioning material of first refractive index, the second layers comprising a material characterized by a transition from ferroelectric phase having second refractive index to nonferroelectric phase having third refractive index different from the first and second indices upon being heated to a characteristic transition temperature, each of the first and second layers having a thickness substantially equal to one-fourth times the ratio of the predetermined wavelength to the respective index of refraction.

RIGHTS OF THE GOVERNMENT

The invention described herein may be manufactured and used by or forthe Government of the United States for all governmental purposeswithout the payment of any royalty.

BACKGROUND OF THE INVENTION

The present invention relates generally to laser hardened materials andstructures, and more particularly to a novel optical filter structurefor selectively blocking laser radiation of predetermined wavelengthwhile passing radiation of other wavelengths.

Optical switching devices comprising transition or switching materialswhich are transparent in one state but which transform to an opaquemetallic state when heated to a characteristic transition temperatureare well developed for applications such as optical filters, modulators,laser output couplers, and the like. These devices are generallycharacterized by a transition from substantial transparency below thecharacteristic transition temperature to substantial opacity above thattemperature.

The present invention comprises an optical reflection filter which issubstantially totally transmissive at substantially all wavelengthsbelow a characteristic temperature, but which becomes reflective uponbeing heated to a ferroelectric phase transition temperature uponabsorption of invasive radiation.

The invention comprises an optical filter structure including alternatelayers of ferroelectric material and another suitable material whichhave substantially equal refractive indices in the unswitched statebelow the characteristic temperature, and which have in the switchedstate an optical thickness substantially equal to one-fourth thewavelength of the invasive radiation to be block by the filter. When thefilter of the invention is heated to the characteristic temperature, therefractive index of the ferroelectric layers changes to a differentvalue which produces in the filter classic quarter wave interference. Afilter according to the invention may therefore be configured to allowan optical system to continuously receive an operational signal andsimultaneously block invasive, potentially destructive, radiation.

It is therefore a principal object of the present invention to providean improved optical filter.

It is a further object of the present invention to provide an opticalfilter for selectively blocking radiation of preselected wavelengthwhile passing radiation of other wavelengths.

It is yet a further object of the invention to provide an optical filterwhich is transparent below a characteristic temperature but whichswitches to a reflective state upon being heated to the characteristictemperature.

It is yet another object to provide an optical filter having fastresponse time.

It is yet a further object of the invention to provide an optical filterhaving low absorption losses prior to switching.

It is yet another object of the invention to provide an optical filterwhich is independent of the wavelength of the impinging radiation priorto switching.

These and other distinguishing features and objects of the presentinvention will become apparent as the detailed description of certainrepresentative embodiments thereof proceeds.

SUMMARY OF THE INVENTION

In accordance with the foregoing principles and objects of the presentinvention, a novel optical filter structure for selectively blockingradiation of predetermined wavelength is described which comprises aplurality of alternate first and second layers deposited on asubstantially transparent substrate, the first layers comprisingmaterial of first refractive index, the second layers comprising amaterial characterized by a transition from ferroelectric phase havingsecond refractive index to nonferroelectric phase having thirdrefractive index different from the first and second indices upon beingheated to a characteristic transition temperature, each of the first andsecond layers having a thickness substantially equal to one-fourth timesthe ratio of the predetermined wavelength to the respective index ofrefraction.

DESCRIPTION OF THE DRAWINGS

The present invention will be more clearly understood from the followingdetailed description of certain representative embodiments thereof readin conjunction with the accompanying drawings wherein:

FIG. 1 is a fragmentary sectional view of a layered structure of thepresent invention.

FIG. 2 is a plot of refractive index versus temperature for bariumtitanate.

FIG. 3 is a plot of reflectance versus relative wave number illustratinga representative reflectance spectrum for a filter according to theinvention.

FIG. 4 is a plot of minimum transmission versus number of layer pairs oftetragonal barium titanate in the structure of the invention.

FIG. 5 is a plot of layer pairs of the structure of the invention versusnonferroelectric refractive index in tetragonal barium titanate.

FIG. 6 is a plot of reflection band width for a representative structureof the invention versus nonferroelectric refractive index for tetragonalbarium titanate.

FIG. 7 is a plot of maximum laser angle versus maximum tolerablefractional spectral shift for the invention.

DETAILED DESCRIPTION

Referring now to FIG. 1 of the drawings, shown therein is a schematicsectional view of a layered structure representative of the opticalfilter of the invention. As shown in FIG. 1, a filter 10 of theinvention comprises a predetermined plurality N of film layers 11 offerroelectric material and alternated with N film layers 12 of othersuitable transparent material, the structure thereby comprising apredetermined number 2N of layers in the form of l layer pairs. Theferroelectric layers 11 comprise a material characterized by atransition from one optically transparent phase having a characteristicindex of refraction to a different optically transparent phase having adifferent index of refraction upon being heated to a characteristicferroelectric-to-nonferroelectric phase transition temperature T_(C).Layers 12 are nontransitioning in the operational temperature range offilter 10. The ferroelectric material layers 11 and nontransitioninglayers 12 are selected to have substantially equal refractive indices inthe unswitched (below T_(C)) state of layers 11 and whose opticalthicknesses are equal to λ_(o) /4 in the switched state of layers 11,where λ_(o) is the wavelength of radiation sought to be blocked byfilter 10. Therefore, when invasive radiation of wavelength λ₀ impingeson the stack of layers 11,12, the stack is heated to T_(C), therefractive index of layers 11 changes to a different value above T_(C),and a quarter wave stack interference filter results.

Filter 10 is preferably constructed of ferroelectric layers 11 of amaterial having a characteristic T_(C) above room temperature and asignificant change of refractive index at the transition temperature.Preferably, the characteristic T_(C) will not be significantly aboveroom temperature. Layers 11 may therefore comprise substantially anytransparent ferroelectric material undergoing the desired change ofrefractive index, such as barium titanate, strontium titanate and mixedalloys thereof; barium niobate, strontium niobate, and mixed alloysthereof; and barium sodium niobate, potassium niobate, bismuthgermanate, and strontium sodium niobate.

Layers 11,12 may be deposited by any process known in the art for suchpurposes, although vacuum techniques, and particularly radio frequencysputtering may be preferable as being the process most reproducible. Therefractive index of a sputtered layer may not be the same as that of thecorresponding bulk crystalline material, but may dependent upon thedegree of crystallinity, substrate temperature during deposition, andoxygen content of the sputtering atmosphere. thus for a given wavelengththe refractive indices of layers 11,12 may be controllable over a fairlywide range which allows close matching of refractive indices of layers11,12 below the characteristic T_(C) of layers 11.

Filter 10 may therefore be fabricated by first (preferably) depositing anontransitioning layer 13 (substantially identical to a layer 12) on asubstrate 15 comprising a transparent insulator or semiconductormaterial. Following deposition of layer 13, alternate layers 11,12 aredeposited to respective preselected thicknesses to produce the quarterwave stack shown in FIG. 1 having l layer pairs. The layer thicknessd_(H) for layers 11 are chosen so that,

    d.sub.H =λ.sub.o /4n.sub.H                          (1)

and d_(L) for layers 12 is,

    d.sub.L =λ.sub.o /4n.sub.L                          (2)

where λ_(o) is the wavelength to be blocked, n_(H) is the refractiveindex of the ferroelectric material above the transition temperature,and n_(L) is the refractive index of the nontransitioning material(layers 12) at substantially all operational temperatures for thefilter. Anti-reflective coatings may be applied on the last appliedlayer at surface 17 or to either surface 18,19 of substrate 15 (e.g., atthe layer 13-substrate 15 interface prior to deposition) to reducereflection losses. Further, an odd number of layers 11,12 may be appliedwithout significant impairment of filter 10 performance if such isdesired for convenience of fabrication or otherwise.

The stepwise change in refractive index of filter 10 may be illustratedby considering an ordinary light ray 21 incident of intensity I_(o) atan angle θ_(o) at surface 17 of filter 10, a portion thereof beingreflected as illustrated at θ_(o) as reflected ray 23 of intensityRI_(o). Transmitted (refracted) ray 25 of intensity TI_(o) is emergentfrom substrate 15 at angle θ_(o) (assuming the index of refraction n_(o)of the medium, e.g., air, near surface 17 is the same as that nearsurface 19), but offset from the direction of incidence of ray 21 by adistance characteristic of the refractive index of filter 10. Thetemperature dependence of the operation of filter 10 for anextraordinary ray is more pronounced than for the ordinary ray, and canbe enhanced by optical illumination to lower the transition temperature.

Nontransitioning layers 12 of filter 10 may be any material which issubstantially transparent over the wavelength range of interest andwhich does not undergo a phase change so as to alter its index ofrefraction in the operational range of filter 10 (e.g., from about -20°C. to about 120° C.) and which preferably has lattice constants likethose of the ferroelectric layers 11 (e.g., BaTiO₃). For example, a50/50 composition of strontium titanate and barium titanate has acharacteristic phase change below -40° C. and a lattice constantsubstantially equal to that of substantially pure barium titanate.Suitable materials for layers 12 may be selected as would occur to onewith skill in the field of the invention.

Referring now to FIG. 2, shown therein is a plot of index of refractionversus temperature for barium titanate (BaTiO₃), a desirable materialfor transition layers 11 of filter 10. An abrupt and significant changeat 31 in refractive index from 2.37 to 2.40 at about 120° C. is evident.The illustrated change in refractive index corresponds to a change incrystal structure of the barium titanate from tetragonal (a=3.99,c=4.035 angstroms at 0° C.) in region 33 below 120° C. to cubic (a=4.01angstroms at 150° C.) in region 35. The tetragonal structure for BaTiO₃is ferroelectric, while the cubic structure is nonferroelectric. Thetetragonal structure us birefringent characteristic of the symmetry ofthe crystal.

Referring now to FIG. 3, shown therein is an illustration of thespectral performance of a typical ferroelectric reflection band filterof the present invention. Plotted in FIG. 3 is the reflectance R of thefilter versus relative wave number ν/ν_(o) where ν_(o) is the frequencyof the radiation the filter is designed to reflect. The maximumreflection R_(2N) and minimum transmission T_(min) are related as,

    T.sub.min +R.sub.2N =1                                     (3)

The width of the central reflection band 37, defining the range ofrelative wave numbers reflected by the filter is defined as BW at halfmaximum of reflection band 37. The minor higher order oscillations 38a-hshown in FIG. 3 on either side of central reflection band 37 are not ofsignificance to the teachings hereof.

Consider an example filter of the invention as illustrated in FIG. 1 isrequired to reflect invasive radiation of wavelength at 0.5 microns.Using the reflective indices given in FIG. 2 the layer thickness d_(H)and d_(L) from Equations (1) and (2) are, respectively, 0.05208 micronsand 0.05274 microns. The minimum transmission of the filter at thecentral reflection band 37 centered on the central frequency ν_(o) (ofthe invasive radiation) is given by:

    T.sub.min =4/(n.sub.H /n.sub.L).sup.l                      (4)

The numerical results of Equation (4) are given in two different formatsin FIGS. 4 and 5. In FIG. 4, T_(min) of the filter (from Equation (4))versus number of pairs l is presented for various values of n_(H) and ann_(L) of 2.37 (characteristic of tetragonal BaTiO₃). In FIG. 5, numberof layer pairs l versus n_(H) is plotted for various desired values ofT_(min). In the present example, i.e., and n_(H) equals 2.40, a value ofT_(min) of 10⁻² may be obtained using 475 layer pairs and of 10⁻³ may beobtained using 655 layer pairs. It is noted that certain manufacturingadvantages are manifest for n_(L) appreciably different from n_(H).

The width at half maximum BW (see FIG. 3) of the central reflection band37 is given by: ##EQU1##

FIG. 6 plots BW versus n_(H) (for cubic BaTiO₃). In the present example(n_(H) =2.40), BW is seen to be less than 0.01. From FIG. 4 it is seenthat a large value for n_(H) is desirable in order to minimize therequired number of layer pairs for a selected value of T_(min). As seenfrom FIG. 6, no practical limitations are necessarily imposed on BW byachieving a larger value of n_(H).

Referring again to FIG. 1, the effects of angle of incidence of invasiveradiation on the shift of the central reflection band can be calculatedas follows. The maximum angle of incidence θ_(max) at which the filterwill operate as intended is given by, ##EQU2## where γ is the maximumtolerable fractional shift of the central reflection band. In theexample discussed above (n_(H) =2.40, n_(L) =2.37), θ_(max) is 13° for apractical value of a γ equal to 0.5. The total field of view 2 θ_(max)is therefore 26° for the example. Achieving a large value for n_(H) inorder to reduce l results in an increase θ_(max). FIG. 7 presentsθ_(max) versus γ for various values of n_(H) according to Equation (6).

The present invention, as hereinabove described, therefore provides anoptical reflection band filter for selectively reflecting a preselectedwavelength upon being heated to a characteristic temperature, whileremaining substantially transparent to all wavelengths below thecharacteristic temperature. The response of the filter is wavelengthindependent prior to switching from the transparent state to thereflecting state. The filter may be constructed to be effective atangles of incidence of invasive radiation up to about 15°. The filterblocks radiation centered about a preselected band centered on apreselected invasive wavelength and, due to the switching mechanismcharacterizing its operation, exhibits minimum absorption losses in theunswitched (transparent) state.

It is understood that certain modifications to the invention asdescribed may be made, as would occur to one with skill in the field ofthe invention, within the scope of the appended claims. Therefore, allembodiments contemplated hereunder and encompassed within the scope ofthe claims have not been shown in complete detail. Other embodiments maybe developed without departing from the spirit of the invention or fromthe scope of the appended claims.

I claim:
 1. An optical filter structure for selectively block radiationof preselected wavelength, comprising:(a) a substantially transparentsubstrate; (b) a plurality of alternate substantially identical firstlayers and substantially identical second layers on said substrate,wherein said first layers comprise a transparent material of preselectedfirst refractive index, and said second layers comprise a materialhaving a substantially transparent ferroelectric phase of secondrefractive index below a characteristic transition temperature and anonferroelectric phase of third refractive index different from saidfirst refractive index and from said second refractive index above saidcharacteristic transition temperature, said first layers having saidfirst refractive index above and below said characteristic transitiontemperature; and (c) wherein the thickness of each of said first layersis equal to one-fourth times the ratio of said preselected wavelength tosaid first refractive index and the thickness of each of said secondlayers is equal to one-fourth times the ratio of said preselectedwavelength to said third refractive index.
 2. The filter structure asrecited in claim 1 wherein said second layers comprise a ferroelectricmaterial having a characteristic transition temperature near roomtemperature.
 3. The filter structure as recited in claim 1 wherein saidsecond layers comprise a ferroelectric material having a characteristictransition temperature higher than room temperature.
 4. The filterstructure as recited in claim 3 wherein said second layers comprise amaterial selected from the group consisting of barium titanate,strontium titanate, barium niobate, strontium niobate, barium sodiumniobate, potassium niobate, bismuth germanate, and strontium sodiumniobate.
 5. The filter structure as recited in claim 1 wherein saidfirst refractive index equals said second refractive index.
 6. Thefilter structure as recited in claim 1 further comprising anantireflection coating on at least one of a surface of the layer mostremote from said substrate and a surface of said substrate.